Metal Chemistry in the Unusual Outflows of the Hypergiant Star VY Canis Majoris (VY CMa): A Focus on Salt

VY Canis Majoris (VY CMa), a hypergiant star, offers an intriguing platform for studying chemistry in the ejecta of massive stars. It experiences intense mass loss, generating asymmetric outflows that create intricate structures comprised of dust and gas, including arcs, knots, and clumps. The material lost in their outflows populate the diffuse interstellar medium. The interplay between gas and dust is important in the stellar ejecta. To explore this phenomenon, ALMA at Band 6 was used to image the refractory molecules NaCl, AlO, AlOH, TiO₂, and, notably, KCl, with resolutions of 0.25" to 1". We combined this data with single-dish observations from the Arizona Radio Astronomy Observatory Submillimeter Telescope to capture total flux. NaCl, common table salt, displays emission closely that surrounds the central star and also traces an ejected cloudlet of gas called the SW Clump, located at a distance of about 180 R_*. Their presence in the SW Clump suggests direct but intact expulsion from the star's photosphere. KCl, a novel discovery in VY CMa, exhibits a similar distribution. Conversely, emission from AlO, AlOH, and TiO₂ exist within 60 R_* of the star, indicating rapid condensation onto the dust grain. The persistence of the alkali halides is a chemical puzzle. NaCl is found in solar system objects such as Mars, Europa, and Io and in protoplanetary disks. Alkali halides, in particular salt, appear to have a unique chemistry that avoids complete condensation, unlike other metal-bearing molecules.